The purpose of this project is to determine the natural history of the recently discovered and isolated treponemalike causative agent of Lyme disease and related disorders (ECM of Europe). The relationship(s) between the spirochete and its various Ixodes spp. tick vectors (I. dammini, I. pacificus, I. scapularis, I. ricinus) is being determined by establishing through conventional as well as transmission and scanning electron microscopy methods a) the development of the spirochete within the ticks, and b) the mode(s) of transmission to vertebrate hosts. In cooperation with outside agencies, tick/spirochete surveys are being conducted to determine prevalence of infected ticks in endemic foci. Similarly, the natural source(s) for infection of ticks is being evaluated first serologically (indirect immunofluorescence) and subsequently through recovery of the spirochete from serologically implicated hosts.